Electroluminescent displays are known as light emitting type electronic display device (ELD). An inorganic electroluminescent element and an organic electroluminescent element (organic EL element) are cited as the constituting element of the ELD. The inorganic EL element has been used as a planar light source, in which high alternative voltage is required for driving the light emitting device.
The organic EL element is an element having a light emitting layer placed between a cathode and an anode, in which electrons and positive holes are injected into the light emitting layer and excitons are generated by recombination of them, and fluorescence or phosphorescence light is emitted on the occasion of quenching of the excitons. Such the device is noted because which can emit light by application of a voltage of several to several tens volts, and has wide viewing angle and high visibility since it is a self light emission type, and is completely solid state thin device suitable for space saving and portable appliance.
However, in an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption. In Japanese Patent No. 3093796, a slight amount of a fluorescent substance has been doped in a stilbene derivative, distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element. Further, there are known such as an element having an organic emitting layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with a slight amount of a fluorescent substance (for example, JP-A S63-264692) and an element having an organic emitting layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye (for example, JP-A H03-255190).
In the case of utilizing emission from an excited singlet as described above, since a generation ratio of a singlet exciton to a triplet exciton is 1:3, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of an external quantum efficiency (ηext) of taking out is said to be 5%.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (M. A. Baldo et al., Nature vol. 395, pp. 151-154 (1998)), researches on materials exhibiting phosphorescence at room temperature have come to be active. For example, it is also disclosed in A. Baldo et al., Nature, vol. 403, No. 17, pp. 750-753 (2000), and U.S. Pat. No. 6,097,147.
It may be possible to realize about four times emission efficiency in principle by an organic EL element employing recently discovered phosphorescent emission in comparison with those employing conventional fluorescent emission. Development of the material therefore and further research and development of the layer arrangement and electrodes are conducted through the world. For example, in such as S. Lamansky et al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001), many compounds mainly belonging to heavy metal complexes such as iridium complexes have been synthesized and studied.
Since the organic EL element is an all solid state element composed of a film of an organic material exhibiting about 0.1 μm in thickness in between two electrodes, and the light emission can be achieved with a relatively low voltage of 2 to 20 volts, the organic EL element is a promising technology for use as the next-generation flat display or lighting device.
It may be possible to realize about four times emission efficiency in principle by an organic EL element employing recently discovered phosphorescent emission in comparison with those employing conventional fluorescent emission. Development of the material therefore and further research and development of the layer arrangement and electrodes are conducted through the world.
The organic EL element is ought to be produced with low cost since it simply composed of an organic layer placed between a transparent electrode and a counter electrode and requires an extremely smaller number of parts than a liquid crystal display which is a representative plain display. However, the liquid crystal display actually leads in performance and cost.
One of a factor is a low productivity as for the cost. The almost of all the organic EL element is produced via so called vapor deposition method in which a layer is formed by a vapor deposition of a low molecular weight material. The vapor deposition method is very advantageous in efficiency and life time because an easily purified low molecular weight compound can be used for the organic EL element, a material with high purity can be easily obtained, and a multi-layered structure can be easily formed. Contrarily a layer forming apparatus is restricted since the deposition is conducted under a condition of high vacuum such as 10−4 Pa or less, and is disadvantageous in such low through-put that it is applicable to a substrate having small format and requires long time to form a plurality of layers. This is problematic in applied to an illumination use or a large format electric display, and this is one of the reason why the organic EL element is not practiced in these applications.
A coating method in which an organic layer is formed via spin coat, ink-jet, printing, spraying can form a layer having large area and is suitable for forming uniform layer having large area.
However, it is preferred plural functional layers are laminated to attain a high emission efficiency as well as ling life simultaneously. There is a method in which an upper layer is formed by coating employing a solvent which does not dissolve the lower layer when a low molecular weight compound is used to form plural layers by employing a coating method. However there is problem to deteriorate the element performance by breaking carrier balance due to generation of turbulence at the interface between layers when the upper layer is coated in the method described above.
Further, it is proposed that a high molecular weight compound is employed as for a material capable of coating. However it is difficult to apply because the high molecular weight compound is difficult to purify in general and a very few impurity causes deterioration of emission life time of the element in the organic EL element particularly.
There have been various technologies to dissolve the above-described problems. Examples include a technology where a constituting layer of an organic electroluminescent element is formed, and then the layer is polymerized, and in which technology bifunctional triphenylamine derivatives having two vinyl groups in their molecules are disclosed, and after the film formation of the compound, a three-dimensionally crosslinked polymer is formed (for example, refer to Patent Document 1); a technology where materials having two or more vinyl groups are incorporated into a plurality of layers, where the polymerization reaction is carried out via irradiation of the ultraviolet rays or heat at a step of forming an organic layer before an cathode is laminated (for example, refer to Patent Document 2); a production technology where a polymerization reaction is allowed to proceed during a film formation by adding an AIBN (azoisobutyronitrile), a radical generating agent, into a mixture of a material having vinyl groups at terminals of a phosphorescent light emitting dopant and a co-monomer similarly having vinyl groups (for example, refer to Patent Document 3); and a production technology where a Diels-Alder reaction is allowed to occur between two molecules within the same layer to result in a linkage (for example, refer to Patent Document 4).
However, there is no sufficient method to dissolve the problem of deterioration of the element due to turbulence between the interface in any technologies described above, and further, the influence of interface turbulence is remarkable in case that one of the layers to be laminated is an emitting layer.    Patent Document 1: JP-A H05-271166    Patent Document 2: JP-A 2001-297882    Patent Document 3: JP-A 2003-073666    Patent Document 4: JP-A 2003-086371